The present emphasis in the development of information storage systems is the capability to store more and more information into a so-called "desk top" sized computer memory system. Those "desk top" sized memory systems which incorporate magnetically recorded hard disc media, such as that used in Winchester disc drive type memory systems, currently have the capacity to store upwards of 20 megabytes of magnetically recorded information. The problem with such memory systems is that by necessity the hard disc media is permanently mounted into the computer. Since the media is not easily removable, the user is limited to whatever portion of the hard disc is remaining for information storage at the time of use. Accordingly, magnetically recorded hard disc media information storage systems are not viewed as a potential solution to increasing information storage capacity.
So-called "floppy" disc memory systems wherein flexible discs, each having a diameter of either 5.25 inches or 3.50 inches, are used as the storage media provides easily removable storage media. However, the problem with such storage systems is that the present storage capacity of information magnetically recorded on a single floppy disc used in such a system has not yet reached a level equal to that of the hard disc, i.e. a single floppy disc media can only store approximately 1 to 2 megabytes of magnetically recorded information.
Systems for storing information which can be accessed through optical devices have received significant attention due to their potential capacity to store substantially more data, i.e. on the order of 400 to 800 megabytes of information, than that available in either magnetically recorded hard disc or floppy disc storage systems. Additionally, the media for use in such optical systems can be of a form similar to that of a so-called floppy disc, that is a media which is easily removable. Unfortunately, major problems continue to plague the development and commercial acceptance of such optical systems, namely the relative slowness by which information can be retrieved compared to magnetic storage systems and the current size restrictions of so-called "desk top" computers.
Consider first the current size restrictions. So-called "desk top" computers have been provided with a number of modular components, particularly including information storage systems, which can be added into the casing of the computer to provide a certain degree of customizing to fit a particular need. Since such components can have any one of a number of sizes, the American National Standards Institute has adopted certain external standard dimensions with regard to such components, which standards are generally referred to as full-height and half-height standards. Since the half-height standard appears to be the most desirable for such modular components, a need exists to develop an optical information storage system which will fit into the halfheight standard. The half-height standard for modular components is as follows: height 1.625 inches; width 5.75 inches; and depth 8.00 inches. The problem with current optical storage systems is that present designs and techniques require components which when assembled easily exceed this size standard. It appear that only a few of the currently available systems include an optical head assemblies, which is only one component of an optical storage system, which would fit into such a size standard.
Consider now the relative slowness by which information can be retrieved in current optical information systems compared to magnetic storage systems. The primary factor contributing to the slow access problem of present optical storage systems is the weight of the optical head assembly. As will be appreciated, the greater the weight of a device for reading from or writing to an optical disc, the more difficult and consequently slower it will be to orient such a device in relation to precise locations on a rotating disc.
Present optical storage systems include those found in video disc or compact disc (CD) players, which are of the read only variety and those which are termed write once read many times (WORM) optical storage systems. Presently the ability to write and read many times to and from an optical disc is primarily limited by the media available for use in such systems. The invention described herein does not address research to provide an optical media which is capable of having information written and erased many times. Nor is the present invention to be limited only to media which currently exists.
In present CD systems an optical disc is rotated about a central axis. A laser beam is projected onto the surface of the disc by means of a lens, a reflecting mirror or beamsplitter, and a projection lens. The laser beam is modulated by the information stored on the optical disc and the modulated light is detected by a photodetector. Output signals from the photodetector are provided to a processor for producing information signals and tracking signals. The source of the laser beam, the lens, the mirror, the projection lens and the detector are collectively referred to as a optical head assembly. The optical head assembly is typically moved radially across the rotating disc in order to access the information stored on the disc.
Since the information to be read or written on an optical disc is contained in narrow tracks, coarse and fine radial movement mechanisms are provided. Until the present invention, it has not been possible, or even considered desirable, to combine coarse and fine movements into one mechanism. The coarse radial movement mechanism typically includes either a pivoting arm or a radially oriented track which moves the optical head assembly radially across many tracks. The fine radial movement mechanism generally operates to move the projection lens either along a radial axis, which causes the projection lens to move between a few adjoining tracks, or along an axis generally perpendicular to the disc, which allows the projection lens to dynamically focus the laser beam on the surface of the disc during operation. Such fine movement mechanisms can be found in the commercially available optical head assemblies sold by the Olympus Corporation of Lake Success, N.Y. (TAOHS Series) or Pentax Teknologies of Broomfield, Colo. (VU-108-02 Series). An example of such an optical head assembly can also be found in U.S. Pat. No. 4,092,529--Aihara et al. The height of these fine movement mechanisms also contributes to the size restriction problem. Reference is also made to the WORM optical storage system shown in Rosch, Winn L., "WORM's for Mass Storage" PC Magazine, Vol. 6, No. 12 (June 23, 1987) pages 135-148.
Since the fine movement mechanism typically causes movement of the projection lens through the use of relatively massive electro-magnets, the overall weight of the optical head assembly is such that movement of the assembly is cumbersome and thus relatively slow. In an attempt to resolve the access time problem, efforts to reduce the optical head assembly have been reported. However, since the fine movement mechanism is still an essential component, the weight and size contributed by such a mechanism remains. For example, Fukui, Y. et al., "New servo method with eccentricity correction circuit" Optical Engineering, Vol. 26, No. 11 (November, 1987) pages 1140-1145 discloses an optical head assembly which describes the combination of an anamorphic prism, a convex lens and a roof shaped prism in an effort to determine data and tracking information signals from a single light beam. Such a combination appears to result in fewer optical components, however, the size and weight of the fine movement mechanism remains.